In previous studies, we have reported that calcium waves among gap junction-coupled glia may form the cellular substrate for spreading depression in vivo. Recently, we have noted that calcium waves initiated in astrocytes in slices can propagate to brain endothelial and meningeal cells; all of these cell types express connexin43, which may allow their mutual heterotypic syncytial interaction through homotypic gap junctions. On this basis, we propose to test the hypothesis that astrocytic calcium waves may thereby invade the brain by propagating along the capillary vasculature, as well as through the astrocytic syncytium. These experiments will test the possibility that endothelial calcium waves may follow the venular endothelium to invade the meningeal vasculature, thereby recruiting both meningeal cells and trigeminal sensory afferents. This proposed pathway, by bypassing and traversing the restrictive barrier of the pia limitans, would permit the recruitment of both the meningeal vasculature and its trigeminal sensory afferents into parenchymal waves of spreading depression. We propose here that this scenario might operationally model the initiation of migraine headache in adults. In parallel experiments, we will also follow-up our recent observation of a steroid-induced accentuation of astrocytic calcium signaling, by asking whether calcium signaling among non-neuronal brain cell types may be modulated by gonadal steroids. In particular, we seek to determine whether the cyclical female hormones estrogen and progesterone potentiate signaling from astrocytes to endothelial cells, and if so, whether the likelihood of meningovascular recruitment into a parenchymal calcium wave is thereby increased. This pathway might account for much of the symptomatology of migraine headache, while steroidal accentuation of calcium signaling might account for the cyclicity of migraine occurrence. The long-distance multicellular calcium signaling pathway that we propose, and its attendant hormonal regulation, suggests immediately testable strategies for its abrogation.